gallium/pipe: Add a GALLIUM_PIPE_SEARCH_DIR override env var

This can be useful if you rsync an install between two machines and the
paths don't perfectly match up.  OpenGL drivers already work fine but
anything which uses pipe-loader has a compile-time path.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7047>

docs/envvars.rst

@@ -391,6 +391,9 @@ Gallium environment variables
 ``GALLIUM_LOG_FILE``
    specifies a file for logging all errors, warnings, etc. rather than
    stderr.
+``GALLIUM_PIPE_SEARCH_DIR``
+   specifies an alternate search directory for pipe-loader which overrides
+   the compile-time path based on the install location.
 ``GALLIUM_PRINT_OPTIONS``
    if non-zero, print all the Gallium environment variables which are
    used, and their current values.

src/gallium/auxiliary/pipe-loader/pipe_loader_drm.c

@@ -98,7 +98,11 @@ get_driver_descriptor(const char *driver_name, struct util_dl_library **plib)
    }
    return &kmsro_driver_descriptor;
 #else
-   *plib = pipe_loader_find_module(driver_name, PIPE_SEARCH_DIR);
+   const char *search_dir = getenv("GALLIUM_PIPE_SEARCH_DIR");
+   if (search_dir == NULL)
+      search_dir = PIPE_SEARCH_DIR;
+
+   *plib = pipe_loader_find_module(driver_name, search_dir);
    if (!*plib)
       return NULL;
 

src/gallium/auxiliary/pipe-loader/pipe_loader_sw.c

@@ -105,7 +105,11 @@ pipe_loader_sw_probe_init_common(struct pipe_loader_sw_device *sdev)
    if (!sdev->dd)
       return false;
 #else
-   sdev->lib = pipe_loader_find_module("swrast", PIPE_SEARCH_DIR);
+   const char *search_dir = getenv("GALLIUM_PIPE_SEARCH_DIR");
+   if (search_dir == NULL)
+      search_dir = PIPE_SEARCH_DIR;
+
+   sdev->lib = pipe_loader_find_module("swrast", search_dir);
    if (!sdev->lib)
       return false;